As disclosed in, for example, U.S. Pat. No. 6,734,496 corresponding to JP-A-H9-266311, a super junction metal oxide semiconductor field-effect transistor (MOSFET) has been proposed that achieves improved breakdown voltage and improved on-resistance. In the super junction MOSFET, a drift region is constructed with n-type drift regions and p-type compartment regions that are alternately arranged. Each of the p-type compartment regions is positioned between adjacent n-type drift regions to form a p-n junction. When the MOSFET is in an ON condition, a drift current flows through the n-type drift regions. In contrast, if the MOSFET is in an OFF condition, a depletion layer spreads out from each p-n junction between the n-type drift region and the p-type compartment region into the n-type drift region. In this case, since the depletion can be accelerated by laterally extending the outermost ends of the depletion region from both longitudinal sides of the p-type compartment region, the p-type compartment region is simultaneously depleted. Consequently, a breakdown voltage of the MOSFET becomes high. Further, on-resistance of the MOSFET can be reduced by increasing impurity concentration of the n-type drift region.
JP-A-2004-72068 and JP-A-2004-134714 disclose a technique for improving an avalanche capability of such a super junction MOSFET. In JP-A-2004-72068 and JP-A-2004-134714, the width or the impurity concentration of the n-type drift region and the p-type compartment region is adjusted so that electric field distribution in the p-n junction can be improved to increase the avalanche capability.
The super junction MOSFET disclosed in U.S. Pat. No. 6,734,496, JP-A-2004-72068, and JP-A-2004-134714 has a planar gate structure. In a super junction MOSFET with a trench gate structure, avalanche breakdown occurs directly below a trench gate. Therefore, an avalanche current flows to a source electrode by way of a channel layer on an outer side wall of a trench and a source layer. Since this avalanche current flow causes a parasitic bipolar transistor action, it is difficult to improve an avalanche capability of a super junction MOSFET with a trench gate structure.